In lithography, there is an ongoing desire to reduce the size of features in a lithographic pattern in order to increase the density of features on a given substrate area. In photolithography, the push for smaller features has resulted in the development of technologies such as immersion lithography and extreme ultraviolet (EUV) lithography, which are however rather costly.
A potentially less costly approach to smaller features that has gained increasing interest is so-called imprint lithography, which generally involves the use of a “stamp” (often referred to as an imprint template) to transfer a pattern onto a substrate. A potential advantage of imprint lithography is that the resolution of the features is not limited by, for example, the emission wavelength of a radiation source or the numerical aperture of a projection system. Instead, the resolution is mainly limited to the pattern density on the imprint template.
Imprint lithography involves the patterning of an imprintable medium on a surface of a substrate to be patterned. The patterning may involve pressing together a patterned surface of an imprint template and a layer of imprintable medium such that the imprintable medium flows into recesses in the patterned surface and is pushed aside by protrusions on the patterned surface. The recesses define pattern features of the patterned surface of the imprint template. Typically, the imprintable medium is flowable as the patterned surface and the imprintable medium are pressed together. Following patterning of the imprintable medium, the imprintable medium is suitably brought into a non-flowable or solid state and the patterned surface of the imprint template and the patterned imprintable medium are separated. The substrate and patterned imprintable medium are then typically processed further in order to pattern or further pattern the substrate. The imprintable medium is typically formed from resist droplets on the surface of a substrate to be patterned.
Compared to conventional lithography, imprint lithography faces the same challenges with respect to accuracy and throughput or yield. As such, it is desirable to have an accurate alignment of an imprint template and a substrate (comparable to the alignment requirements of a mask or reticle and a wafer in conventional lithography) before the imprint template is pressed into the imprintable medium. In order to obtain such accurate alignment, an accurate positioning, substantially free of external disturbances, of the imprint template and the substrate is desired. In order to realize this, both the imprint template and the substrate can be mounted on a vibration isolation system thus creating an isolated environment, substantially free of vibrations such as floor vibrations. Such a vibration isolation system is often characterized by a comparatively low stiffness and damping resulting in a cut-off frequency of e.g. 1 Hz or less.
Similar to conventional lithography, an imprint lithographic apparatus should, for economic reasons, be capable of processing a sufficient number of wafers or substrates per unit of time without compromising the accuracy or yield. In order to achieve such a sufficient throughput (e.g. expressed as the number of wafers processed per hour), the time expiring between two consecutive imprint steps should be kept as brief as possible.
If a vibration isolation system is used in a conventional lithographic apparatus were to be implemented in an imprint lithographic apparatus, it would be difficult to realize a sufficient throughput. This is due to the particular nature of the imprint process, which uses relatively high forces exerted on the substrate and imprint template.
In view of this, it is desirable to provide a vibration isolation system for an imprint lithographic apparatus which enables improvement of the apparatus's throughput, substantially without adversely affecting the accuracy of the imprint process.